Automatic frequency control for carrier-wave receivers



A. R. ALTER Jan. 19, 1954 AUTOMATIC FREQUENCY CONTROL FOR CARRIE-WAVE RECEIVERS Filed June 25, 1949 2 Sheets-Sheet l 1N V EN TOR.

A. R. ALTER Jan. 19, 1954 AUTOMATIC FREQUENCY CONTROL FOR CARRIER-WAVE RECEIVERS 2 ysheets-sheet 2 Filed June 25, 1949 mkbhmi.

Patented Jan. 19, 1954 STATES PTENT ltd? OFFICE AUTOMATIC FREQUENCY CONTROL FOR CARRIER-'WAVE 'RECEIVERS Application June'25, 1949SerialNo. 101,465

(Cl. Z50-,20)

'13 Claims. l

The present invention relates Ato automatic f requency control circuits, especially as incorporated in television receivers.

Automatic frequency .control (AFC) has been extensively used in conjunction with audio or sound receivers of the superheterodyne type. Such receivers generally include a discriminator which acts to develop a direct-current control potential from the intermediate-frequency (IF) energy when the latter shifts in frequency from an assigned value. In addition to the discriminator unit, a reactance tube may be employed which is associated with the resonant tank circuit of the local oscillator. The control potential from the discriminator is applied :to the reactance tube so as to vary the reactance placed across the oscillator tank circuit, such that, with a proper selection of components, the oscillator frequency is maintained `at the correct value vfor each particular setting of the receiver tuning mechanism.

The above-described method of irequencycontrol has been utilized in connected with television receivers of the type employing a separate set of tuning coils for each lof the thirteen signal channels. In a preferred design, the antenna coil and the R-F amplifier coil for each channel are huilt as a single assembly. The mixer .coil and the local oscillator coil are also formed as a unit, so that these two assemblies together fpro- Vide a set of four coils for each one of the -television channels. A so-oalled lturret switch, or channel selector, accommodates interchangeably any eight of these sets of coils, and the receiver is accordingly adapted to receive the full number of transmitting stations allotted under present standards to any one particular area.

When the desired antenna coil, R-'F coil, local oscillator coil, and mixer coil have been connected to the antenna lead-in by a proper setting of the channel selector knob, the signal .from v,the antenna is transferred to the R-F amplifier circuit. l-lere the signal is amplified and then impedance-coupled into the mixer circuit, where it hetercdynes with the output of the oscillator .to produce the predetermined video and audio intermediate frequencies. The output of the local oscillator is inductively coupled to the grid circuit of the mixer, from which tube the signal is fed into the video and audio channels 'of the receiver. An automatic frequencl7 control circuit, of the nature described above, is incorporated with the local oscillator in such a manner that a control signal kfrom the discriminator in the audio section of the receiver is applied :to

govern the action of the oscillators, raising or lowering its frequency as necessary to maintain at all times the desired intermediate frequency. This compensates for any frequency drift in the incoming signal, Aas well as for any small frequency drifts that may arise Within the receiver itself.

In a system of the nature described above, the discriminator in the audio section .of the receiver must be designed to provide an adequate AFC voltage voutput over the full bandwidth through which the IF signal may vary as the receiver .is tuned to a particular channel. This bandwidth, or range of the audio discriminator necessary for AFC control, is'much greater than that normally required to effect a .demodulation of the audio signal. For example, in commercial receivers employing the AFC system just described, the required bandwidth of the audio discriminator may be of the order of `500 kc., even through the maximum `sound .deviation is in the order of iZ- kc. :Such a requirement for the audio discriminator reduces its effective sensitivity and may require that the gain of the audio channel be vraised by the addition of one or more stages of I-F amplification. lThis expedient, however, increases the complexity as wel1 as the overall cost of the television receiver.

In accordance with one feature of the present invention, the bandwidth of the audio discriminator is made only as Wide as is necessary for proper sound demodulation, this being vonly slightly inexcessof '5G kc. inthe example given.

An incluctance-capacitance network is provided y in conjunction with the AFC circuit, and a positive voltage is developed across the capacitance of this LC lnetwork during the reiatively short period of time that the turret tuning device, or channel selector, is being manually moved from one channel to another. .Following this manipulation, and when vthe desired Ychannel has been tuned in, the -capacitance of the LC network `is in effect shunted by the inductance, andan oscillatory action occurs which causes the potential across the capacitor to take the form of a damped sinusoidal Wave-that is, the voltage on the Vcapacitor drops to zero, v.rises in a negative direction, `and then finally yreturns to zero. If this varying capacitor ,potential is applied Ato the grid of the reactance tube which governs the `frequency of Ithe Ilocal oscillator, the latter may be Ymade to similarly vary in frequency.

During this frequency variation, the Il? signal in the audio channel will sweep yacross the response curve of the discriminator. This results inthe development of an AFC control voltage from the discriminator which is of suicient amplitude to lock in the local oscillator to its proper frequency.

The above cycle of operation will take place each time that the channel selector is shifted from one channel to another. After the abovedescribed lock-in action takes place, the energy in the LC circuit drops to zero, and the eiect of this circuit on the normal AFC action is of no further consequence.

One object of the present invention, therefore, is to provide an improved automatic frequency control circuit.

Another object of the invention is to provide, in an automatic frequency control arrangement for television receivers of the type wherein the AFC control voltage is derived from the output of the discriminator in the audio circuit of the receiver, means whereby the bandwidthof such discriminator need only be sufciently wide to adequately reproduce the audio signal.

A further object of the invention is to provide, in a television receiver of the type described, an AFC pull-in circuit which is energized as a function of the manual operation of the receiver tuning device, thereby to provide an output voltage which is effective to bring the local oscillator signal within the lock-in range of the AFC circuit.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawings, in which:

Fig. 1 is a schematic representation of a television receiver employing an automatic frequency control circuit in accordance with the present invention;

Fig. 2 illustrates separately the pull-in circuit of Fig. 1 in such a manner that its operation may be more readily described;

Figs. 3 and 4 are graphical representations useful in describing the .operation of the cir'- cuit of Fig. 1.

To the foregoing general ends it is a feature of the present invention to provide, in an AFC system for television receivers of the type in which the output of a local oscillator is heterodyned with a television signal containing both audio and video information, and in which the receiver includes in the audio section thereof a frequency discriminator the bandwidth of which exceeds only slightly the normal bandwidth of the audio I-F signal (the latter being subject to uctuations beyond the range of the discriminator during a tuning of the receiver from one channel to another), means for deriving from the discriminator an AFC voltage whenever the audio I-F signal departs from a center I-F frequency but remains within the range of the discriminator, means for developing a control potential as a function of the tuning of the receiver from one channel to another, and a circuit for applying the control potential thus developed to bring the audio I-F signal within the frequency range of the discriminator in the event that it does not then lie within such range.

Referring first to Fig. 1, there is shown in a more or less schematic fashion a television receiver of the super-heterodyne type incorporating a preferred form of the present invention. Inasmuch as a number of the components of this receiver are of a conventional nature, they have been illustrated by appropriatelydabelled blocks in order to simplify the drawing. In general, the receiver includes an antenna I from 4 which incoming signals are fed to a channel selector switch I2. The function of the latter is to tune the receiver to the particular television broadcasting station which the operator desires to receive. The channel-selecting portions of this switch I2 form no part of the present invention, and hence it is not believed necessary to set forth these elements in detail. For a full description of the selector mechanism, however, reference is made to a copending United States patent application of L. H. Zepp et al., Serial No. 736,435, led March 22, 1947, which matured into U. S. Patent 2,545,681 on March 20, 1951. It will only be mentioned herein that the switch l2 includes a pair of rotatable turret members I3 and I4 each of which is secured to a shaft I6 in any suitable manner. These turret members are adapted for rotational movement about the axis of shaft I6 in response to the manual manipulation of a control knob I8.

The turret member I3 carries a plurality of sets of coil windings, two of which sets are identiiied by the reference numerals 20 and 22. It will of course be understood that additional sets of coils are carried by member I3 (one set for each of eight channels, for example), but the remainder have been omitted from the drawing for the sake of clarity. Each set of coils includes an oscillator winding and a mixer winding, these two windings being so positioned that there is mutual inductive coupling therebetween.

The turret member I4 likewise supports a plurality of sets of coils which are aligned with respect to the inductive elements carried by the turret member I3. Each set of windings associated with the member I4 includes an R-F coil, an input (or antenna) coil, and a link (or coupling) coil. In Fig. l these three coils in one representative set carried by the turret member I4 are respectively designated by the reference numerals 24, 26 and 28, respectively.

The turret members I3 and I4 are provided with a plurality of contact buttons to which the various coil terminals are connected. Certain of these contact buttons are indicated in Fig. 1 by the reference numeral 30. It will be appreciated that the various sets of coils may be brought progressively into a predetermined angular position in which the buttons 30 bear against corresponding stationary contacts 32 each of which forms a terminal for a conductor leading to a particular circuit portion of the television receiver. For convenience of illustration, the movable contact buttons 3B carried by the turret members I3 and I4 have been depicted in Fig. l by means of arrowheads, and the stationary contact elements 32 by means of circles. It will be further understood that actuation of the control knob I8 to rotate the shaft I6 acts to bring the sets of movable contacts 30 progressively into registry with the stationary contacts 32.

In the above-described manner the antenna IE is connected to one of the antenna coils the particular coil selected being dependent upon the angular position of the selector switch I2. By means of the link coil 28, the signal energy in the antenna coil 25 is transferred to the grid of an R-F amplifier tube 34 through the R-F coil 24. The three coils 24, 26 and 28 of eac'. set therefore comprise what may be termed an antenna coupling network.

Each set of inductive elements (such as 26) carried by the turret member I3 inclu-ies :1r-.i oscillator coil 36 and a mixer coil 38. The oscillator coil 3S is connected in the grid-plate circuit of an oscillator tube lill, while `the vmixer coil 3S is connected between the anode of the R-F ampliher tube 3G and the control grid of a mixer tube 2. The oscillator coil 3S is tuned to resonance by both the associated plate-tocathode capacity and grid-to-cathode capacity of the tube which capacities appear in series across the coil.

The radio-frequency output of the R-F amplifier tube 3f. is applied to the control grid of the mixer, or rst detector, tube 52 by way of a coupling network which includes the coil 38 and a capacitor 1M. This coupling network may be tuned to resonance by means of a trimmer condenser fit. The signal generated by the oscillator tube le is injected into the grid circuit of the mixer by way of the mutual inductance between the coils 36 and 3B. As a result of this action, there is developed in the output circuit of the mixer tube #i2 an intermediate frequency (I-F) signal the frequency of which is equal to the difference between the frequencies of the two input signals. This I-F output of the mixer tube i2 is applied to an input -F amplifier 48.

The output of the -E amplifier 8 will consist of two I-F carriers, one of which is modulated with the picture, or video, signals, and the other of which is modulated with the accompanying sound, or audio, signals. The latter are amplified in one or more audio -F amplifier stages 55.3, while the former are ainplied by one or more video I-F amplifier stages 52. The sound -F signals are impressed upon a discriminator, or second detector, 54 which acts to develop a detected audio-frequency signal in the usual manner. The latter is then amplified in the audio amplifier 5S and reproduced by some suitable device such as a loudspeaker 58.

The video -F signals, on the other hand, are applied to the picture second detector t, which cle-modulates the video signals. The picture information thus derived is amplified in the video frequency amplifier 62 and employed to modulate the electron scanning beam of a cathode ray tube, or other image-reproducing device, t4. It will be understood that the synchronizing signal information in the composite television signal is separated from the output of the video amplifier 62 by some suitable means, which may be of standard design and which therefore has not been illustrated. ft is likewise assumed that a suitable l1-C. restoring circuit is utilized in the video channel of the receiver to supply the proper background illumination for the image reproduced by the tube 65.

Since the present invention is not concerned with the foregoing circuits, except insofar as they cooperate with the elements of applicants invention to be hereinafter described, a further and more detailed description thereof is not believed necessary, although reference is again made to the mentioned Zepp et al. application for a more extended discussion.

In order to maintain, at all times, intermediate frequency carriers of predetermined frequency in both the audio and video channels of the television receiver, an automatic frequency control, or AFC, circuit is utilized. This circuit makes use of a control voltage derived from the discriminator 5ft in the sound channel. For example, if the carrier frequency output of the amplier Eil departs from a predetermined frequency, a control voltage will be obtained from the discriminator et the magnitude of which will vary as a function of the degree of departure of the I-F signal from this frequency, and the polarity of which will depend upon the direction of this frequency departure. A discriminator circuit operating in accordance with these principles is set forth in United States Patent No. 2,121,103, issued June 2l, i938, to S. W. Seeley, and hence a detailed description of such apparatus will not be given. It is deemed sufficient to state that a direct-current voltage (AFC bias) is derivedfrom the I-F energy, and that the polarity and magnitude of this bias is dependent on the sense and ainount of frequency shift of the I-F carrier from its preassigned frequency. The specific construction of the discriminator 54 forms no part of the present invention, and any type of circuit may be used as long as it is capable of converting ra frequency shift vin the I-.F signal into a representative D.-C. voltage variation.

The output of discriminator 5P is passed through an AFC filter network et the function of which is to smooth out variations in the AEC control voltage so that it will be suitable for application as a bias potential to the control electrode 6s of a reactance control tube lil.

The reactance control tube l, as shown in Fig. l, is effectively connected in parallel relation with the oscillator tube dil. Hence tube l is so related to the oscillator tank circuit, including the coil 36 (plate-to-cathode and grid-to-cathode capacitances of tube lf3 being considered as part of this tank circuit) that the tube 'i6 places an effective inductive reactance across this tank circuit. In other Words, tube 'it simulates or refleets a reactance of predetermined sign across the tank circuit of the oscillator tube t. The magnitude of this reactance is a function of the mutual conductance of 'tube l0. ff the AFC voltage applied to the grid S2 is positive, the mutual conductance of tube 'lc is increased. This has the same effect as if the inductance of coil 38 were increased in value thereby causing the frequency of the oscillator it to decrease. Conversely, if the AFC voltage is negative, the mutual conductance of tube 'lc is decreased, and this effectively decreases the inductance of coil 36 and hence increases the local oscillator frequency. In this manner, a control signal from the discriminator 5d governs the action of the reactance control tube HG so as to raise or lower the frequency'of local oscillator tube lil as necessary to maintain at all times the preassigned intermediate frequency. This compensates for any frequency drift in the incoming signal received by antenna if), as well as for any frequency drift that may arise within the receiver itself.

In the circuit as described above, it will Abe seen that the discriminator 5d must be designed to provide an adequate AFC control voltage output over the full frequency band through which the I-F signal may vary as the television receiver is tuned from one channel to another through manipulation of the channel selector knob i8. If the discriminator bandwidth is not great enough toinclude these frequency swings, no AFC control voltage output will be produced from the discriminator 5e when the I-F signal is outside the discriminator range.

Accordingly, in commercial receivers embodying the above-described AFC arrangement, the bandwidth of the discriminator 54% is in the neighborhood of 500 kc., and is greatly in excess of the bandwidth required for proper sound demodulation, since the peak-:to-peak frequency deviation 'due to modulation is in the neighborhood of .50.

kc. The employment of a discriminator having such a wide bandwidth results in a detected sound output of relatively low level unless an excessively large number of I-F amplier stages are employed preceding the discriminator 54. As above brought out, the use of additional I-F stages is costly and in addition renders the receiver unnecessarily complex.

In accordance with one feature of the present invention, the bandwidth of the discriminator 54 is made only as wide as is necessary for proper sound demodulation, such, for example, as 100 kc. Normally, this would not result in the development of an adequate AFC control voltage when the receiver is switched from one channel to another, inasmuch as the intermediate frequency may be outside the response curve of the discriminator 54 and, if so, no appreciable control potential will be generated. To meet this possibility, the present invention makes use of a pull-in circuit 12. This circuit 'I2 consists of a capacitor 14, a choke coil 'I6 having distributed resistance, and a resistor i8, all of which elements are connected in series between ground and the terminal 80 of a source of positive potential.

As will now be described, a varying voltage is developed across capacitor 14, and this voltage is applied through a coupling capacitor 82 to the control electrode 68 of the reactance control tube in. A point intermediate the resistor i8 and the choke coil 'IG (and which point is designated by the reference character A in Fig. l) is normally connected to ground through a pair of contacts carried by the turret member I4 of the channel selector switch I2. As shown in Fig. l, this point A on the pull-in circuit 'l2 is connected to one of the stationary switch contacts 32, this particular contact being designated as 32a. The corresponding movable contact 36a is connected directly to another movable contact 3D1), as illustrated. The stationary contact 3217, which is associated with 'the movable contact 3G11, is grounded. Thus, when the channel selector switch I2 is so positioned that the apparatus of Fig. l is adapted to receive signals transmitted over a particular television channel, then the point A in the pull-in circuit l2 is connected to ground. However, when the channel selector switch I2 is being manually actuated by means of the knob I8, and switch I2 is between channels, point A is not grounded since there is then no electrical engagement between the contacts 36a and 32a, or between the contacts 30h and 32h.

The channel selector switch I2 is adapted to snap into each selected position. The shaft I6 is indexed in such a manner that each step-bystep movement of the shaft brings different sets of coils into a position in which engagement is made between them and the xed contacts 32. This indexing means is fully set forth in the copending Zepp et al. application referred to above, and it will only be mentioned that such means may include a cam member 84 having a plurality of detent areas (eight in the present example). This cam member 64 is, of course, fixed to the shaft I6, and cooperates with a lever 86 connected to a plate (not shown) by a pin 88. Lever 85 carries a roller 99 which is urged into contact with the cani member 84 as by means of a spring (not shown) and cooperates with the aforesaid detent areas to insure that the turret members I3 and I4 will be properly indexed in positions such that the desired movable contacts 30 are in registry with the nxed contacts 32.

In describing the operation of the circuit of Fig. l, let it nrst be assumed that the elements of the channel selector switch I2 are in the positions shown. As such, point A is grounded, and (a suitable period of time having elapsed since the knob I8 was last manipulated) condenser 'I4 has no charge appearing thereon. The grounding of point A of course precludes current from flowing into the condenser 74 from the positive potential source connected to the terminal 80.

However, when the channel selector knob I8 is actuated as to tune the television receiver from one channel to another, contacts 30a-32a and 30h-32h Aare momentarily opened. This permits condenser I4 to be charged through the choke coil 'I6 and the resistor '13, and the magnitude of this charge on condenser 'I4 will reach the full voltage of the potential source during the time the switch contacts of the tuning device are open.

When the channel selector knob I8 has thus been actuated and the desired television channel switched in, the upper end of resistor 16 (point A in Fig. 1) is again grounded through the contacts 30a-39h and 32a- 3222. The positive voltage developed on capacitor 'I4 (during the interval when the channel selector l2 was between channel positions) will now drop to Zero, overshoot negative, and eventually return to zero due to the oscillatory nature of the pull-in circuit '12. Since this varying voltage on capacitor I4 is applied through the coupling capacitor 82 to the grid 66 of the reactance control tube lil, the frequency of the oscillator circuit controlled by the reactance tube will vary in a similar manner.

In order to bring out more clearly the characteristics of the potential applied to the reactance tube circuit, reference is made to Fig. 2 of the drawings, in which the pull-in circuit 'I2 of Fig. 1 is shown separately. In Fig. 2 the voltage appearing at point A in Fig. l is represented as ei and its amplitude by E. Capacitors i4 and E2 are designated as C and C1, respectively. The choke coil 'I6 has its inductance indicated by L, while its distributed resistance is set forth in dotted lines and identied by the reference character R.

As shown in Fig. 2, ei consists of a step of voltage of magnitude E caused by the grounding of point A in Fig. l at time t=0 when contacts Ella-36h and 32a- 321) are initially closed. The output voltage end) may be expressed as The D.C. term in this equation is prevented from appearing on the grid SB of the reactance control tube El by the presence of the coupling capacitor C1. Therefore the actual control voltage ex becomes the waveform of which is illustrated in Fig. 3 of the drawings.

It will now be seen that the voltage output of the pull-in circuit i2 will be a damped sinusoidal wave having a maximum amplitude E equal to the charge developed on condenser lli during the interval of time when the channel selector switch I2 1s in a position between channels. This varying voltage output from the pull-in circuit 'I2 as above brought out, is applied to the control electrode 68 of the reactance control tube 7U. The result of rthis mode of operation is to cause the frequency of the local oscillator tube 49 to vary in a similar manner, and hence the intermediate frequency carrier in the audio channel will also sweep through a range having limits both above and below the assigned intermediate frequency.

The maximum excursions of this frequency variation may lie beyond the lock-in range of the discriminator-reactance tube circuit during the first few cycles of oscillation of the pull-in circuit 1?., However, during each of these initial cycles the signal in the audio channel will sweep through the AFC circuit lock-in range in the manner shown by Fig. 4 of the drawings. Accordingly, an AFC control voltage will be developed in the discriminator 5i! during each such sweep period. rhis control voltage is applied to the control electrode 5S of the reactance control tube lil so that the latter may be effective to alter the resonant frequency of the oscillator tank circuit. Thus, while wide variations in frequency may occur during the switching action of the channel selector l2, nevertheless the pull-in circuit 'i2 acts to bring the output of the local oscillator d within the lock-in range of the discriminator-reactance tube network in a cyclic manner, and, while the audio I-F signal is within this lock-in range, lan effective AFC voltage output is developed in the discriminator d to bring about a stabilized circuit operation.

The voltage output of the discriminator 54 in Fig. 1 is relatively higher' for a given frequency deviation from center I-F than is the case with the standard, wide-band discriminator employed conventional circuits. IThis is apparent from an inspection of Fig. 4, in which the slope of the discriminator curve for Fig. 1 is shown as being considerably greater than the slope of the usual discriminator characteristic.

It will be apparent from the foregoing discussion that the AFC voltage developed by the discriminator Ell of Fig. l varies in a cyclic manner in response to operation of the pull-in circuit i2. Accordingly, attention should be given, in the design of the system, to ensure that the filter network 6e is capable of transmitting this alternating component in the output of the discriminator. The frequency at which the AFC control voltage changes must not be in excess of that permitted by the time constant of the AFC filter, since otherwise no effective control bias will be applied to the grid 68 of the reactance control tube. This is brought out by the showing of Fig. 4, where the slope of the dotted line represents the time constant of the AFC filter 55. It will be appreciated that the slope of the audio I-F signal waveform (within the lock-in range of the discriminator) should not be greater than the slope of the dotted line representing the time constant of the filter if an AFC voltage is to be developed. ln case the oscillator frequency does vary initially at a higher rate than can be passed by the iilter circuit 6E, several cycles of oscillation of the pull-in circuit 'l2 may occur before locloin is obtained. However, this is of no material consequence, since the additional fraction of a second required for lock-in under such conditions will not be objectionable to the operator of the television receiver.

lt will be seen that the lock-in action of the discriininator-reactance tube circuit is otherwise independent of the slope of the I-F signal wave, since, although the local oscillator 48 is swept at an arbitrary rate, it nevertheless must ultimately enter the lock-in region of the AFC detectorreactance tube circuit, and once in that region is placed under the control of that circuit. It is only necessary, as above brought out, that the rate of sweep of the I-F signal be such that the time constant of the AFC iter circuit 86 is not exceeded.

Following the pull-in action ofthe circuit 12, the voltage on condenser i4 falls to zero, and the pull-in circuit T2 is completely inactive until such time as the channel selector switch I2 is again actuated to another channel position.

The following values have been found suitable for the components of the pull-in circuit '12, although they are of course to be regardedas being merely exemplary:

Resistor 78 47,000 ohms Choke 76 2.5 millihenries Capacitor 74 .1 microfarad Capacitor 32 .01 microfarad Potential source 80 +6 volts If desired, point A in the pull-in circuit 'l2 may be connected to one of the stationary contacts 32 other than 32a. For example, either of the stationary contacts may be used which lead to the antenna lil, since these are grounded through the link coil 23.

Having thus described my invention, I claim:

l. In an AFCsystem for television receivers of the type in which the output of a local oscillator is heterodyned with a television signal containing both audio and video information so as to develop video and audio I-Fv signals, and wherein a manually-actuatable channel selector switch having a plurality ofy indexed positions is employed to tune the receiver to a desired channel, said receiver further including in the audio section thereof a frequency discriminator, said audio I-F signal being subject to fluctuations in frequency beyond the range of said discriminator during the tuning of said receiver from one channel to another: thev combination of means for deriving from said discriminator an AFC voltage whenever the said audio I-F signal departs from a center I-F frequency but lies within the discriminator frequency range; a capacitor; an inductor; a source of potential; a circuit connecting said capacitor, said inductor, and said Source of potential in series relation; a set of switch contacts carried by said channel selector switch in normally-closed condition when said selector switch is in an indexed position; a circuit, including said set of switch contacts, shunting said source of potential; means responsive to the manual actuation of said channel selector switch from one of its indexed positions to another to momentarily open said set of contacts and hence remove the shunt from said potential source, whereby said potential source will charge said capacitor through said inductor; means responsive to the subsequent closing of said contacts following the manual actuation of said channel selector switch to again shunt said potential source, whereby an oscillatory voltage is developed in said inductance-capacitance circuit; and means for applying the oscillatory voltage thus developed to control the operation of said local oscillator.

2. In an automatic frequency control system for a carrier-wave receiver of the type in which the output of a local oscillator is heterodyned with a received carrier wave to develop 'in response thereto an I-F carried signal having a nominal center frequency, and wherein a manually-actuable channel selector device having a plurality of indexed positions is employed to tune the receiver selectively to any one of a plurality of preselected channels, said receiver further including a frequency discriminator responsive to said I-F carrier signal, said I-F carrier signal being subject to fortuitous departures from said nominal center frequency beyond the effective range of said discriminator during the tuning of said receiver from one channel to another, said receiver further including a reactance control tube circuit operatively coupling said frequency discriminator and said local oscillator: the combination comprising a capacitor; an inductor; a source of potential; means connecting said capacitor, said inductor and said source of potential in closed series circuit relation; a set of switch contacts carried by said channel selector device, said contacts being in closed relation when said channel selector device is in an indexed position; a circuit, including said set of switch contacts, shunting said source of potential; means responsive to the manual actuation of said channel selector device from one of its indexed positions momentarily to open said contacts and hence remove the shunt from said source of potential, whereby said capictor is charged through said inductor; means responsive to the subsequent closing of said contacts following the manual actuation of said channel selector device to again shunt said source of potential, whereby a transient oscillatory voltage is developed across the resulting parallel combination o1" said inductor and said capacitor; and means for applying said transient oscillatory voltage to said reactance tube circuit whereby to effect a transient perturbation in the frequency of said local oscillator sufficient to bring said I-F carrier signal within the said eiective range of said discriminator,

3. In an automatic frequency control system for a carrier Wave receiver of the type in which the output of a local oscillator is heterodyned with a received carrier wave to develop in response thereto an I-F carrier signal having a nominal assigned frequency, and wherein a manually-actuable channel selector ldevice having a plurality of indexed positions is employed to tune the receiver selectively to any one of a plurality of preselected channels, said receiver further including a frequency discriminator responsive to said I-F carrier signal being subject to fortuitous departures from said nominal assigned frequency beyond the effective range of said discriminator during the tuning of said receiver from one channel to another, said receiver further including a reactance control tube circuit operatively coupling said frequency discriminator and said local oscillator: the lcombination comprising frequencymodulating means operatively associated with said channel selector device and with said oscillator, said frequency-modulating means including oscillatory means, exclusive of the hereinbefore recited apparatus, for developing a transient oscillation in response to actuation of said channel selector device from one to another of its indexed positions, said frequency modulating means being effective to produce in response to said transient oscillation a transient oscillatory perturbation in the frequency of said local oscillator suicient to bring said I-F carrier signal within the said eiective range of said discriminator.

ALBERT R. ALTER.

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